Lubricating oil composition

ABSTRACT

Use of a lubricating oil composition comprising, or made by admixing, a major amount of oil of lubricating viscosity, and minor amounts of (A) a dispersant additive composition and (B) a detergent additive composition, wherein the oil composition gives a sulfated ash content of at most 1.0 mass %; has a total base number (TBN) of 4 to 9.5; has at least 0.08 mass % of nitrogen derived from the dispersant additive composition, based on the mass of the oil composition; and has at least 25 mmol of soap per 1000 g of the oil composition derived from the detergent additive composition, to improve the cleanliness of pistons in an engine.

The present invention relates to improved additive and lubricating oil compositions, such as multigrade lubricants, particularly those demonstrating better piston cleanliness.

Lubricating oil compositions (or lubricants) for the crankcase of internal combustion engines are well-known and it is also well-known for them to contain additives (or additive components) to enhance their properties and performance.

Environmental concerns have led to continued efforts to reduce the particulate emissions of vehicular internal combustion engines, particularly compression ignited (diesel) internal combustion engines. One technology being used to reduce particulate emissions of diesel engines is the particulate trap, which is to be incorporated into all passenger car and heavy duty diesel vehicles designed to comply with the requirements of Euro IV emissions legislation. When lubricant is consumed during use in the engine, ash derived from metal-containing additives in the lubricant, primarily from metal-containing detergents and antiwear agents, accumulate in the particulate trap. This ash cannot be purged without removing the trap from the engine and cleaning either via washing or blowing the ash from the particulate trap with compressed air. Ash allowed to accumulate in the particulate trap may cause an increase in pressure behind the trap (back pressure). If this back pressure becomes severe, internal exhaust gas recirculation may occur with a resulting loss of fuel economy and eventual engine failure. Because lubricants require acid neutralization (provided by detergents), and to a lesser extent wear protection (provided by ZDDP), metal-containing additives that form ash upon use in engines cannot simply be removed. The drive to lower sulfated ash in lubricants affects at least the available detergents, particularly overbased detergents, that can be included, which consequently impacts on piston cleanliness, particularly in high temperature diesel engines. High temperature piston cleanliness in diesel engines may measured by the VWTDi test (according to CEC L-78-T-99 procedure). This test also provides an indication of the degree of sticking of the piston rings (referred to as “ring-sticking”).

Thus, the formulators have to carefully control the choice and amount of additives and basestock in a lubricating oil composition to achieve the required performance whilst satisfying the limits set by governmental and automotive bodies.

U.S. Pat. No. 5,102,566 describes low sulphated ash lubricating oil compositions, which contain ashless dispersants, oil-soluble anti-oxidants and oil-soluble dihydrocarbyldithiophosphates.

EP-A-1167497 describes a lubricating oil composition having low P content, low sulphated ash content and low sulphur content.

EP-A-1 266 955 describes using an ester basestock to improve the piston cleanliness properties. While EP-A-1 087 008 describes a way of improving “ring-sticking” performance by provision of molybdenum-containing additive components to the lubricating oil composition.

However, Applicants have now surprisingly found that the combination of an increased amount of soap and an increased amount of dispersant provides improved piston cleanliness in lubricating oil compositions having at most 1.0 mass % sulphated ash.

Accordingly, in a first aspect, the present invention provides a lubricating oil composition comprising, or made by admixing, a major amount of an oil of lubricating viscosity, and minor amounts of (A) a dispersant additive composition and (B) a detergent additive composition, wherein the oil composition gives a sulfated ash content of at most 1.0, such as in the range 0.3 to 0.9, preferably 0.5 to 0.7, mass %; has a total base number (TBN) of 4 to 9.5, such as 5 to 9, preferably 6 to 8.5; has at least 0.08, for example, 0.085 to 0.115, preferably 0.09 to 0.10, mass % of nitrogen derived from the dispersant additive composition, based on the mass of the oil composition; and has at least 25, especially at least 28 or 30, such as at most 35, millimoles (mmol) of soap per 1000 g of the oil composition derived from the detergent additive composition.

The data contained in the specification demonstrate that the use of the increased amounts of soap and dispersant unexpectedly improves the performance of lubricating oil compositions that give less than 1.0 mass % of ash. Further, a preferred ratio of the amount of (A) a dispersant additive composition, based on ppm of nitrogen, in the lubricating oil composition to the amount of (B) a detergent additive composition, based on mmol of soap per 100 g of the oil composition, in the lubricating oil composition is from 22:1 to 46:1, especially from 25:1 to 40:1, such as 27:1 to 30:1. This allows for controlling the debits associated with dispersant additives (e.g. degradation of elastomer seals) and detergent additives, especially salicylate-based additives (e.g. ability to control soot).

In a second aspect, the present invention provides a method of lubricating a compression-ignited internal combustion engine comprising operating the engine and lubricating the engine with a lubricating oil composition of the first aspect.

In a third aspect, the present invention provides a method of improving piston cleanliness and reducing the ring-sticking tendencies of a compression-ignited internal combustion engine comprising adding to the engine a lubricating oil composition of the first aspect.

In a fourth aspect, the present invention provides a combination comprising the crankcase of a compression-ignited internal combustion engine, preferably having a specific power output of 25 kW/litre or greater, and a lubricating oil composition of the first aspect.

In a fifth aspect, the present invention provides the use of (1) a dispersant additive composition in an amount that provides at least 0.085 mass % of nitrogen and (2) a detergent additive composition in an amount that provides at least 25 mmol of soap per 1000 g of the oil composition, in a lubricating oil composition, which oil composition gives a sulfated ash of most 1.0 mass % and has a TBN of 4 to 9.5, to improve the piston cleanliness in an internal combustion engine.

In a sixth aspect, the present invention provides an additive concentrate for preparing a lubricating oil composition comprising an oleaginous carrier fluid, a dispersant additive composition and a detergent additive composition, in such a proportion as to provide a lubricating oil composition as defined in the first aspect when the oil composition contains 10 or 13.5 to 30, preferably 16 to 27, such as 18 to 25, mass %, based on the mass of the oil composition, of additives, said additives excluding viscosity modifier and pour point depressant additives.

The features of the invention will now be discussed in more detail as follows:

Lubricating Oil Composition

The lubricating oil compositions of the present invention are for lubricating the crankcase of an internal combustion engine, preferably a compression-ignited (diesel) engine, more preferably a compression-ignited passenger vehicle engine. Crankcase lubricating oil compositions for a diesel application, in particular for passenger vehicles, have to be specifically formulated to meet the performance requirements in such an application.

It is preferred that lubricating oil compositions of the invention are multigrade oil compositions having a viscometric grade of SAE 5W-X or SAE 0W-X, where X represents 20 and 30; the characteristics of the different grades can be found in the SAE J300 classification.

In another embodiment of the present invention, the lubricating oil compositions of the first aspect have a NOACK volatility of at most 15, such as less than 13, preferably less than 11, mass %, as determined according to CEC L-40-A-93. The NOACK volatility of the lubricating oil composition is generally not less than 4, such as not less than 5.

Further, the lubricating oil compositions of the invention preferably have less than 0.09, such as less than 0.08, preferably 0.01 to 0.07, in particular 0.03 to 0.06, mass % of phosphorus, preferably derived from one or more zinc dithiophosphate additives, based on the mass of the oil composition.

Independently of the other embodiments, the sulfur content of lubricating oil compositions of the invention is preferably at most 0.25, more preferably at most 0.2, such as 0.05 to 0.15, mass %, based on the mass of the oil composition.

The lubricating oil composition may also have a molybdenum content of at most 300, preferably in the range 10 to 200, especially 50 to 175, ppm by mass, based on the mass of the oil composition.

Also, a boron-containing additive may be present in the lubricating oil composition. In such an event, the amount of boron in the oil composition is preferably at most 150, preferably in the range 10 to 100, especially 25 to 75, ppm by mass, based on the mass of the oil composition.

The amount of phosphorus, sulfur, molybdenum and boron are determined according to method ASTM D5185; “TBN” is Total Base Number as measured by ASTM D2896; the amount of nitrogen is determined according to method ASTM D4629; and sulfated ash is measured according to method ASTM D874.

The lubricating oil composition preferably satisfies at least the performance requirements of ACEA B2-98, more preferably at least the ACEA B 1-98, such as at least the ACEA B3-98, especially at least the ACEA B4-98, for light duty diesel engines.

Oil of Lubricating Viscosity

The oil of lubricating viscosity is the major liquid constituent of a lubricating oil composition. The oil of lubricating viscosity includes (a) oil added to an additive concentrate or additive package, and (b) any oil present in an additive concentrate or additive package.

The oil lubricating viscosity can be a synthetic or mineral oil selected from the group consisting of Group I, II, III, IV and V basestocks, and any mixtures thereof.

Basestocks may be made using a variety of different processes including but not limited to distillation, solvent refining, hydrogen processing, oligomerization, esterification, and rerefining.

American Petroleum Institute (API) 1509 “Engine Oil Licensing and Certification System” Fourteenth Edition, December 1996 states that ail basestocks are divided into five general categories:

Group I basestocks contain less than 90% saturates and/or greater than 0.03% sulfur and have a viscosity index greater than or equal to 80 and less than 120;

Group II basestocks contain greater than or equal to 90% saturates and less than or equal to 0.03% sulfur and have a viscosity index greater than or equal to 80 and less than 120;

Group III basestocks contain greater than or equal to 90% saturates and less than or equal or 0.03% sulfur and have a viscosity index greater than or equal to 120;

-   -   Group IV basestocks are polyalphaolefins (PAO); and

Group V basestocks contain all other basestocks not included in Group I, II, III or IV.

Group IV basestocks, i.e. polyalphaolefins (PAO), are generally hydrogenated oligomers of an alpha-olefin, the most important methods of oligomerization being free radical processes, Ziegler catalysis, cationic, and Friedel-Crafts catalysis.

Group V basestocks in the form of esters are preferred and also tend to be commercially available. Examples include polyol esters such as pentaerythritol esters, trimethylolpropane esters and neopentylglycol esters; diesters; C₃₆ dimer acid esters; trimellitate esters, i.e. 1, 2; 4-benzene tricarboxylates; and phthalate esters, ie. 1,2-benzene dicarboxylates. The acids from which the esters are made are preferably monocarboxylic acids of the formula RCO₂H where R represents a branched, linear or mixed alkyl group. Such acids may, for example, contain 6 to 18 carbon atoms.

Preferably the oil of lubricating viscosity is selected from any one of Group I to V basestocks and any mixture thereof, provided that the oil contains at most 0.1, such as at most 0.05, more preferably 0.005 to 0.03, mass % of sulfur, based on the mass of the oil.

Especially preferred is an oil of lubricating viscosity comprising a Group III basestock, advantageously in an amount of at least 20, such as at least 40, more preferably in the range from 55 to 90, mass %, based on the mass of the oil composition.

In a preferred embodiment, the oil of lubricating viscosity comprises a Group III basestock and a Group V basestock in the form of an ester. The amount of Group V basestock in the form of an ester is preferably at most 15, such as 0.5 to 15, more preferably 1 or 2 to 15, especially 3 to 15, more especially 3 to 10, advantageously 3 to 8, such as 5 to 8, mass %, based on the mass of the oil composition. Group I, Group II or Group IV basestock or any mixture thereof may also be present, in a minor amount, in the oil of lubricating viscosity as a diluent or carrier fluid for the additive components and additive concentrate(s) used in preparing the lubricating oil compositions of the invention.

More preferably, the oil of lubricating viscosity consists essentially of Group III basestocks and Group V basestocks in the form of an ester, but may contain minor amounts, such as at most 25, such as at most 20, preferably at most 10, advantageously at most 5, mass %, based on the mass of the total basestock, of other basestocks, such as Group I, Group II or Group IV basestock or any mixture thereof.

The test methods used in defining the above groups are ASTM D2007 for saturates; ASTM D2270 for viscosity index; and one of ASTM D2622, 4294, 4927 and 3120 for sulfur.

Dispersant Additive Composition

Dispersants (or dispersant additives), such as ashless (i.e. metal-free) dispersants hold solid and liquid contaminants, resulting from oxidation during use, in suspension and thus preventing sludge flocculation and precipitation or deposition on metal parts; they comprise long-chain hydrocarbons, to confer oil-solubility, with a polar head capable of associating with particles to be dispersed. A noteworthy group is hydrocarbon-substituted succinimides.

Generally, ashless dispersants form substantially no ash on combustion, in contrast to metal-containing (and thus ash-forming) detergents. Borated metal-free dispersants are also regarded herein as ashless dispersants. “Substantially no ash” means that the dispersant may give trace amounts of ash on combustion, but amounts which do not have practical or significant effect on the performance of the dispersant.

A dispersant additive composition may contain one or more dispersants.

The ashless, dispersants of the present invention comprise an oil soluble polymeric long chain backbone having functional groups capable of associating with particles to be dispersed. Typically, such dispersants have amine, amine-alcohol or amide polar moieties attached to the polymer backbone, often via a bridging group. The ashless dispersant may be, for example, selected from oil soluble salts, esters, amino-esters, amides, imides and oxazolines of long chain hydrocarbon-substituted mono- and polycarboxylic acids or anhydrides thereof; thiocarboxylate derivatives of long chain hydrocarbons; long chain aliphatic hydrocarbons having polyamine moieties attached directly thereto; and Mannich condensation products formed by condensing a long chain substituted phenol with formaldehyde and polyalkylene polyamine. Suitable sispersants include, for example, derivatives of long chain hydrocarbyl-substituted carboxylic acids, in which the hydrocarbyl group has a number average molecular weight tends of less than 15,000, such as less than 5,000; examples of such derivatives being derivatives of high molecular weight hydrocarbyl-substituted succinic acid. Such hydrocarbyl-substituted carboxylic acids may be derivatised with, for example, a nitrogen-containing compound, advantageously a polyalkylene polyamine or amine-alcohol or amide or ester. Particularly preferred dispersants are the reaction products of polyalkylene amines with alkenyl succinic anhydrides. Examples of specifications disclosing dispersants of the last-mentioned type are U.S. Pat. Nos. 3,202,678, 3,154,560, 3,172,892, 3,024,195, 3,024,237, 3,219,666, 3,216,936 and BE-A-662 875.

The dispersant may comprise a polyisobutenyl succinimide prepared using high vinylidene polyisobutene (PIB), for example one in which greater than 80% of the terminal olefin groups have only hydrogen moieties. The total chlorine content of the oil composition of the present invention when such dispersants are used should be below 50 ppm as measured by XRF.

The dispersant(s) of the present invention are preferably non-polymeric (e.g. are mono- or bis-succinimides).

The dispersant(s) of the present invention may optionally be borated. Such dispersants can be borated by conventional means, as generally taught in U.S. Pat. Nos. 3,087,936, 3,254,025 and 5,430,105. Boration of the dispersant is readily accomplished by treating an acyl nitrogen-containing dispersant with a boron compound such as boron oxide, boron halide boron acids, and esters of boron acids, in an amount sufficient to provide from about 0.1 to about 20 atomic proportions of boron for each mole of acylated nitrogen composition.

An ashless succinimide or a derivative thereof, obtainable from a polyisobutenylsuccinic anhydride produced from polybutene and maleic anhydride by a thermal reaction method using neither chlorine nor a chlorine atom-containing compound, is a preferred dispersant.

Alternatively, or in addition, dispersancy may be provided by polymeric compounds capable of providing viscosity index improving properties and dispersancy, such compounds are known as a dispersant viscosity index improver additive or a multifunctional viscosity index improver. Such polymers differ from conventional viscosity index improvers in that they provide performance properties, such as dispersancy and/or antioxidancy, in addition to viscosity index improvement.

Dispersant olefin copolymers and dispersant polymethacrylates are examples of dispersant viscosity index improver additives. Dispersant viscosity index improver additives are prepared by chemically attaching various functional moieties, for example amines, alcohols and amides, on to polymers, which polymers preferably tend to have a number average molecular weight of at least 15,000, such in the range from 20,000 to 600,000, as determined by gel permeation chromatography or light scattering methods. The polymers used may be those described below with respect to viscosity modifiers. Therefore, amine molecules may be incorporated to impart dispersancy and/or antioxidancy characteristics, whereas phenolic molecules may be incorporated to improve antioxidant properties. A specific example, therefore, is an inter-polymer of ethylene-propylene post grafted with an active monomer such as maleic anhydride and then derivatized with, for example, an alcohol or amine. In the event a dispersant viscosity modifier is used in the present invention, the nitrogen content of the lubricating oil composition also includes that derived from the dispersant viscosity modifier. An example of a dispersant viscosity modifier is Hitec® 5777, which is manufactured and sold by Ethyl Corp.

EP-A-24146 and EP-A-0 854 904 describe examples of dispersants and dispersant viscosity index improvers, which are accordingly incorporated herein.

Advantageously, the dispersant additive composition contains one or more dispersants, preferably a borated and non-borated dispersant.

Detergent Additive Composition

A detergent (or detergent additive) reduces formation of piston deposits, for example high-temperature varnish and lacquer deposits, by keeping finely divided solids in suspension in engines; it may also have acid-neutralising properties. A detergent comprises metal salts of organic acids, which are referred herein as soaps or surfactants.

A detergent has a polar head, i.e. the metal salt of the organic acid, with a long hydrophobic tail for oil solubility. Therefore, the organic acids typically have one or more functional groups, such as OH or COOH or SO₃H, for reacting with a metal, and a hydrocarbyl substituent. A detergent may be overbased, in which case the detergent contains an excess of metal in relation to the stoichiometric quantity needed for the neutralisation of the organic acid. This excess is in the form of a colloidal dispersion, typically metal carbonate and/or hydroxide, with the metal salts of organic acids in a micellar structure.

Examples of organic acids include sulfonic acids, phenols and sulfurised derivatives thereof, and carboxylic acids including aromatic carboxylic acids.

Phenols may be non-sulfurized or, preferably, sulfurized. Further, the term “phenol” as used herein includes phenols containing more than one hydroxyl group (for example, alkyl catechols) or fused aromatic rings (for example, alkyl naphthols) and phenols which have been modified by chemical reaction, for example, alkylene-bridged phenols and Mannich base-condensed phenols; and saligenin-type phenols (produced by the reaction of a phenol and an aldehyde under basic conditions).

Preferred phenols are of the formula

where R represents a hydrocarbyl group and y represents 1 to 4. Where y is greater than 1, the hydrocarbyl groups may be the same or different.

The phenols are frequently used in sulfurized form. Details of sulfurization processes are known to those skilled in the art, for example, see U.S. Pat. No. 4,228,022 and U.S. Pat. No. 4,309,293.

In the above formula, hydrocarbyl groups represented by R are advantageously alkyl groups, which advantageously contain 5 to 100, preferably 5 to 40, especially 9 to 12, carbon atoms, the average number of carbon atoms in all of the R groups being at least about 9 in order to ensure adequate solubility in oil. Preferred alkyl groups are nonyl (e.g., tripropylene) groups or dodecyl (e.g., tetrapropylene) groups.

As indicated above, the term “phenol” as used herein includes phenols which have been modified by chemical reaction with, for example, an aldehyde, and Mannich base-condensed phenols.

Aldehydes with which phenols may be modified include, for example, formaldehyde, propionaldehyde and butyraldehyde. The preferred aldehyde is formaldehyde. Aldehyde-modified phenols suitable for use in accordance with the present invention are described in, for example, U.S. Pat. No. 5,259,967 and WO 01/74751.

Mannich base-condensed phenols are prepared by the reaction of a phenol, an aldehyde and an amine. Examples of suitable Mannich base-condensed phenols are described in GB-A-2 121 432.

In general, the phenols may include substituents other than those mentioned above. Examples of such substituents are methoxy groups and halogen atoms.

A preferred phenol is a sulfurised derivative thereof.

Sulfonic acids are typically obtained by sulfonation of hydrocarbyl-substituted, especially alkyl-substituted, aromatic hydrocarbons, for example, those obtained from the fractionation of petroleum by distillation and/or extraction, or by the alkylation of aromatic hydrocarbons. The alkylaryl sulfonic acids usually contain from about 22 to about 100 or more carbon atoms. The sulfonic acids may be substituted by more than one alkyl group on the aromatic moiety, for example they may be dialkylaryl sulfonic acids. Preferably the sulfonic acid has a number average molecular weight of 350 or greater, more preferably 400 or greater, especially 500 or greater, such as 600 or greater. Number average molecular weight may be determined by ASTM D3712.

Another type of sulfonic acid which may be used in accordance with the invention comprises alkyl phenol sulfonic acids. Such sulfonic acids can be sulfurized.

Carboxylic acids include mono- and dicarboxylic acids. Preferred monocarboxylic acids are those containing 8 to 30, especially 8 to 24, carbon atoms. (Where this specification indicates the number of carbon atoms in a carboxylic acid, the carbon atom(s) in the carboxylic group(s) is/are included in that number). Examples of monocarboxylic acids are iso-octanoic acid, stearic acid, oleic acid, palmitic acid and behenic acid. Iso-octanoic acid may, if desired, be used in the form of the mixture of C8 acid isomers sold by Exxon Chemical under the trade name “Cekanoic”. Other suitable acids are those with tertiary substitution at the α-carbon atom and dicarboxylic acids with 2 or more carbon atoms separating the carboxylic groups. Further, dicarboxylic acids with more than 35 carbon atoms, for example, 36 to 100 carbon atoms, are also suitable. Unsaturated carboxylic acids can be sulfurized.

A preferred type of carboxylic acid is an aromatic carboxylic acid. The aromatic moiety of the aromatic carboxylic acid can contain heteroatoms, such as nitrogen and oxygen. Preferably, the moiety contains only carbon atoms; more preferably the moiety contains six or more carbon atoms; for example benzene is a preferred moiety.

The aromatic carboxylic acid may contain one or more aromatic moieties, such as one or more benzene rings, either fused or connected via alkylene bridges.

The carboxylic moiety may be attached directly or indirectly to the aromatic moiety. Preferably the carboxylic acid group is attached directly to a carbon atom on the aromatic moiety, such as a carbon atom on the benzene ring.

More preferably, the aromatic moiety also contains a second functional group, such as a hydroxy group or a sulfonate group, which can be attached directly or indirectly to a carbon atom on the aromatic moiety.

Preferred examples of aromatic carboxylic acids are salicylic acids and sulfurised derivatives thereof, such as hydrocarbyl substituted salicylic acid and derivatives thereof.

Processes for sulfurizing, for example a hydrocarbyl-substituted salicylic acid, are known to those skilled in the art.

Salicylic acids are typically prepared by carboxylation, for example, by the Kolbe-Schmitt process, of phenoxides, and in that case, will generally be obtained, normally in a diluent, in admixture with uncarboxylated phenol.

Preferred substituents for oil-soluble salicylic acids are alkyl substituents. In alkyl-substituted salicylic acids, the alkyl groups advantageously contain 5 to 100, preferably 9 to 30, especially 14 to 20, carbon atoms. Where there is more than one alkyl group, the average number of carbon atoms in all of the alkyl groups is preferably at least 9 to ensure adequate oil-solubility.

The metal detergent may be neutral or overbased; such terms are known in the art. A detergent additive composition may comprise one or more detergent additives, which can be a neutral detergent, an overbased detergent or a mixture of both.

Total Base Number (TBN) of detergents range from 15 to 600.

The detergents of the present invention may be salts of one type of organic acid or salts of more than one type of organic acids, for example hybrid complex detergents.

A hybrid complex detergent is a detergent in which the basic material, e.g. colloidal metal carbonate, within the detergent is stabilised by metal salts of more than one type of organic acid. It will be appreciated by one skilled in the art that a single type of organic acid may contain a mixture of organic acids of the same type. For example, a sulfonic acid may contain a mixture of sulfonic acids of varying molecular weights. Such an organic acid composition is considered as one type. Thus, complex detergents are distinguished from mixtures of two or more separate detergents, an example of such a mixture being one of an overbased calcium salicylate detergent with an overbased calcium phenate detergent.

The art describes examples of overbased complex detergents. For example, International Patent Application Publication Nos. WO 97/46643/4/5/6 and 7, which are incorporated herein in respect of the description and definition of the hybrid complex detergents, describe hybrid complexes made by neutralising a mixture of more than one acidic organic compound with a basic metal compound, and then overbasing the mixture. Individual basic material of the detergent are thus stabilised by a plurality of organic acid types. Examples of hybrid complex detergents include calcium phenate-salicylate-sulfonate detergent, calcium phenate-sulfonate detergent and calcium phenate-salicylate detergent.

EP-A-0 750 659 describes a calcium salicylate phenate complex made by carboxylating a calcium phenate and then sulfurising and overbasing the mixture of calcium salicylate and calcium phenate. Such complexes may be referred to as “phenalates”.

A detergent additive composition may contain two or more detergents, for example, an alkali metal, such as sodium, detergent, and an alkaline earth metal, such as calcium and/or magnesium, detergent. For the avoidance of doubt, the detergent additive composition may also comprise an ashless detergent, i.e. a non-metal containing detergent, typically in the form of an organic salt of an organic acid, in which case the soap corresponds to the salt of the organic acid and the soap derived from such a detergent also contributes to the amount of defined soap in the lubricating oil composition of the present invention. The detergents are preferably metal containing and Group 1 and Group 2 metals are preferred as metals in the detergents, more preferably calcium and magnesium, especially calcium.

Preferably the detergent composition comprises at least one overbased metal detergent, irrespective of whether the detergent contains metal salts of one type of organic acid or metal salts of more than one type of organic acid.

Detergent additive compositions comprising, preferably consisting essentially of, at least one metal detergent based on one or more organic acids not containing sulfur, e.g., carboxylic acid, salicylic acid, alkylene bridged phenols and Mannich base-condensed phenol, are preferred. Especially, salicylate-based detergent have been found to particularly effective. Therefore, s detergent additive composition comprising only metal, preferably calcium, salicylate-based detergents, whether neutral or overbased, are advantageous, such as an overbased calcium salicylate.

The detergent additive composition preferably contains two or more detergents, preferably at least one detergent having a TBN greater than 150 and at least one detergent having a TBN of at most 150.

Preferably at most 35, such as 5 to 30, preferably 10 to 25, % of the mmol of soap is derived from one or more detergents having a TBN greater than 150.

Applicant has found that a detergent additive composition consisting of a calcium salicylate having a TBN of 150 to 200 and a calcium salicylate having a TBN of at most 80 is preferred. Preferably the amount of the calcium salicylate having a TBN of 150 to 200 in the detergent additive composition is such as to contribute at most 35, such as 5 to 30, preferably 10 to 25, % of the mmol of soap per 1000 g of the oil composition to the lubricating oil composition.

In another embodiment, a detergent additive composition consisting of at least one metal salicylate, preferably a calcium salicylate, more preferably a calcium salicylate having a TBN of at most 150, such as at most 100, preferably at most 80, is particularly effective for high temperature piston cleanliness.

In the instance where a composition comprises a detergent and one or more co-additives, then the detergent may be separated from the co-additives, for example, by using dialysis techniques and then the detergent may be analysed as described above to determine the metal ratio. Background information on suitable dialysis techniques is given by Amos, R. and Albaugh, E. W. in “Chromatography in Petroleum Analysis” Altgelt, K. H. and Gouw, T. H., Eds., pages 417 to 421, Marcel Dekker Inc., New York and Basel, 1979.

Means for determining the amount of soap are known to those skilled in the art. EP-A-0 876 449 describes methods for determining the number of moles of a calcium salt of an organic acid, which disclosure is incorporated herein.

A skilled person can also calculate the amount of soap in the final lubricating oil composition from information concerning the raw materials (e.g. amount and type of organic acids) used to make the detergent(s) and from information concerning the amount of detergent(s) used in the final oil composition. Analytical methods (e.g. potentiometric titration and chromatography) can also be used to determine the amounts of soap.

It will be appreciated by a skilled person in the art that the methods to determine the amount of metal salts of organic acids (also known as soap) are at best approximations and that differing methods will not always give exactly the same result; they are, however, sufficiently precise to allow the practice of the present invention.

Additive Concentrate

An additive concentrate constitutes a convenient means of handling two or more additives before their use, as well as facilitating solution or dispersion of the additives in lubricant compositions. When preparing a lubricant composition that contains more than one type of additive (sometimes referred to as “additive components”), each additive may be incorporated separately. In many instances, however, it is convenient to incorporate the additives as an additive concentrate (a so-called additive “package” (also referred to as an “adpack”)) comprising two or more additives.

In the preparation of the lubricant oil compositions, it is common practice to introduce additives therefore in the form of additive concentrate(s) containing the additives. When a plurality of additives are employed it may be desirable, although not essential, to prepare one or more additive concentrates (also known as additive packages) comprising the additives, whereby several additives, with the exception generally of viscosity modifiers, multifunctional viscosity modifiers and pour point depressants, can be added simultaneously to the oil of lubricating viscosity to form the lubricating oil composition. Dissolution of the additive concentrate(s) into the lubricating oil may be facilitated by diluent or solvents and by mixing accompanied with mild heating, but this is not essential. The additive concentrate(s) will typically be formulated to contain the additive(s) in proper amounts to provide the desired concentration in the final formulation when the additive concentrate(s) is/are combined with a predetermined amount of oil of lubricating viscosity. If required, the viscosity modifiers, multifunctional viscosity modifiers and pour point depressants are then separately added to form a lubricating oil composition.

Examples of other additives include rust inhibitors, anti-wear agents, anti-oxidants, corrosion inhibitors, friction modifiers, pour point depressants, anti-foaming agents, viscosity modifiers and surfactants.

An additive concentrate may contain 1 to 90, such as 10 to 80, preferably 20 to 80, more preferably 40 to 70, mass % based on active ingredient, of the additives, the remainder being an oleaginous carrier or diluent fluid (for example, an oil of lubricating viscosity). The final lubricating oil composition may typically contain 5 to 40 mass % of the additive concentrate(s).

The amount of additives in the final lubricating oil composition is generally dependent on the type of the oil composition, for example, a heavy duty diesel engine lubricating oil composition preferably has 10 to 40, more preferably 15 to 35, such as 25 to 30, mass % of additives (including any diluent fluid), based on the mass of the oil composition. A passenger car engine lubricating oil composition, for example, a gasoline or a diesel engine oil composition, tends to have a lower amount of additives, for example 10 or 13.5 to 30, preferably 16 to 27, such as 18 to 25, mass % of additives, based on the mass of the oil composition. The amounts expressed above exclude viscosity modifier and pour point depressant additives.

Generally the viscosity of the additive concentrate is higher than that of the lubricating oil composition. Typically, the kinematic viscosity at 100° C. of the additive concentrate is at least 50, such as in the range 100 to 200, preferably 120 to 180, mm²s⁻¹ (or cSt).

Thus, a method of preparing a lubricating oil composition according to the present invention can involve admixing an oil of lubricating viscosity and one or more of additives or additive concentrates that comprises two or more of additives and then, admixing other additive components, such as viscosity modifier, a multifunctional viscosity modifier and pour point depressant.

Viscosity index improvers (or viscosity modifiers) impart high and low temperature operability to a lubricating oil and permit it to remain shear stable at elevated temperatures and also exhibit acceptable viscosity or fluidity at low temperatures.

Suitable compounds for use as viscosity modifiers are generally high molecular weight hydrocarbon polymers, e.g., polyisobutylene, copolymers of ethylene and propylene and higher alpha-olefins; polyesters, such as polymethacrylates; hydrogenated poly(styrene-co-butadiene or -isoprene) polymers and modifications (e.g., star polymers); and esterified poly(styrene-co-maleic anhydride) polymers. Oil-soluble viscosity modifying polymers generally have number average molecular weights of at least 15,000 to 1,000,000, preferably 20,000 to 600,000, as determined by gel permeation chromatography or light scattering methods. The disclosure in Chapter 5 of “Chemistry & Technology of Lubricants”, edited by R. M. Mortier and S. T. Orzulik, First edition, 1992, Blackie Academic & Professional, is incorporated herein. The VM used may have that sole function, or may be multifunctional.

Friction modifiers include boundary additives that lower friction coefficients and hence improve fuel economy. Examples are oil soluble amines, amides, imidazolines, amine oxides, amidoamines, nitrites, alkanolamides, alkoxylated amines and ether amines and polyol esters, esters of polycarboxylic acids and include glycerol monoesters of higher fatty acids, for example glycerol mono-oleate; butane diol esters of dimerized unsaturated fatty acids; oxazoline compounds; and ethoxylated tallow amine and ethoxylated tallow ether amine. Molybdenum-containing compounds are also examples of friction modifiers. Conventionally, one or more organic friction modifiers are used in an amount of 0.1 to 0.5, such as 0.2 to 0.4, mass %, based on the mass of the oil composition.

Anti-wear agents reduce friction and excessive wear and are usually based on compounds containing sulfur or phosphorus or both. Dihydrocarbyl dithiophosphate metal salts are frequently used as anti-wear and antioxidant agents. The metal may be an alkali or alkaline earth metal, or aluminum, lead, tin, molybdenum, manganese, nickel or copper. The zinc salts (ZDDP) are most commonly used in lubricating oil in amounts of 0.1 to 10, preferably 0.2 to 2 wt. %, based upon the total weight of the lubricating oil composition. They may be prepared in accordance with known techniques by first forming a dihydrocarbyl dithiophosphoric acid (DDPA), usually by reaction of one or more alcohol or a phenol with P₂S₅ and then neutralizing the formed DDPA with a zinc compound. For example, a dithiophosphoric acid may be made by reacting mixtures of primary and secondary alcohols having 1 to 18, preferably 2 to 12, carbon atoms. Alternatively, multiple dithiophosphoric acids can be prepared where the hydrocarbyl groups on one are entirely secondary in character and the hydrocarbyl groups on the others are entirely primary in character. To make the zinc salt any basic or neutral zinc compound could be used but the oxides, hydroxides and carbonates are most generally employed. Commercial additives frequently contain an excess of zinc due to use of an excess of the basic zinc compound in the neutralization reaction.

ZDDP provides excellent wear protection at a comparatively low cost and also functions as an antioxidant. Preferably a zinc dithiophosphate composition comprising one or more zinc dithiophosphate, which composition especially contains a mixture of primary and secondary alkyl groups, wherein the secondary alkyl groups are in a major molar proportion, such as at least 60, advantageously at least 75, more especially at least 85, mole %, based on the amount of alkyl groups, is useful in the present invention. Preferably a zinc dithiophosphate composition has 90 mole % secondary alkyl groups and 10 mole % primary alkyl groups.

Anti-oxidants increase the composition's resistance to oxidation and may work by combining with and modifying peroxides to render them harmless by decomposing peroxides or by rendering an oxidation catalyst inert. They may be classified as radical scavengers (e.g. sterically hindered phenols, secondary aromatic amines, and organo-copper salts); hydroperoxide decomposers (e.g. organo-sulfur and organophosphorus additives); and multifunctionals. Such anti-oxidants (or oxidation inhibitors) include hindered phenols, aromatic amine compounds, alkaline earth metal and metal-free alkylphenolthioesters having preferably C₅ to C₁₂ alkyl side chains, ashless alkylene bridged phenols, phosphosulfurized and sulfurized hydrocarbons, phosphorous esters, metal and metal-free thiocarbamates & derivatives thereof, oil soluble copper compound as described in U.S. Pat. No. 4,867,890, and molybdenum containing compounds. In the practice of the present invention, the use or otherwise of certain anti-oxidants may confer certain benefits. For example, in one embodiment it is preferred that an anti-oxidant composition comprising a secondary aromatic amine and a hindered phenol with an ester group is used.

Preferably an antioxidant composition comprising an aromatic amine, such as diphenylamine and a hindered phenol compound, such as 3,5-bis(alkyl)-4-hydroxyphenyl carboxylic acid esters, e.g. IRGANOX® L135 as sold by Ciba Specialty Chemicals, is useful. Usually, one or more antioxidants are used in an amount of 0.1 to 0.8, such as 0.2 to 0.6, preferably 0.3 to 0.5, mass %, based on the mass of the oil composition.

The molybdenum-containing compounds, preferably molybdenum-sulfur compounds, useful in the present invention may be mononuclear or polynuclear. In the event that the compound is polynuclear, the compound contains a molybdenum core consisting of non-metallic atoms, such as sulfur, oxygen and selenium, preferably consisting essentially of sulfur.

To enable the molybdenum-sulfur compound to be oil-soluble or oil-dispersible, one or more ligands are bonded to a molybdenum atom in the compound. The bonding of the ligands includes bonding by electrostatic interaction as in the case of a counter-ion and forms of bonding intermediate between covalent and electrostatic bonding. Ligands within the same compound may be differently bonded. For example, a ligand may be covalently bonded and another ligand may be electrostatically bonded.

Preferably, the or each ligand is monoanionic and examples of such ligands are dithiophosphates, dithiocarbamates, xanthates, carboxylates, thioxanthates, phosphates and hydrocarbyl, preferably alkyl, derivatives thereof. Preferably, the ratio of the number of molybdenum atoms, for example, in the core in the event that the molybdenum-sulfur compound is a polynuclear compound, to the number of monoanionic ligands, which are capable of rendering the compound oil-soluble or oil-dispersible, is greater than 1 to 1, such as at least 3 to 2.

The molybdenum-sulfur compound's oil-solubility or oil-dispersibility may be influenced by the total number of carbon atoms present among all of the compound's ligands. The total number of carbon atoms present among all of the hydrocarbyl groups of the compound's ligands typically will be at least 21, e.g., 21 to 800, such as at least 25, at least 30 or at least 35. For example, the number of carbon atoms in each alkyl group will generally range between 1 to 100, preferably 1 to 40, and more preferably between 3 and 20.

Examples of molybdenum-sulfur compounds include dinuclear molybdenum-sulfur compounds and trinuclear molybdenum-sulfur compounds.

An example of a dinuclear molybdenum-sulfur compound is represented by the formula:

where R₁ to R₄ independently denote a straight chain, branched chain or aromatic hydrocarbyl group having 1 to 24 carbon atoms; and X₁ to X₄ independently denote an oxygen atom or a sulfur atom. The four hydrocarbyl groups, R₁ to R₄, may be identical or different from one another.

In a preferred embodiment, the molybdenum-sulfur compound is an oil-soluble or oil-dispersible trinuclear molybdenum-sulfur compound. Examples of trinuclear molybdenum-sulfur compounds are disclosed in WO98/26030, WO99/31113, WO99/66013, EP-A-1 138 752, EP-A-1 138 686 and European patent application no. 02078011, each of which are incorporated into the present description by reference, particularly with respect to the characteristics of the molybdenum compound or additive disclosed therein.

Preferably, the trinuclear molybdenum-sulfur compounds are represented by the formula Mo₃S_(k)E_(x)L_(n)A_(p)Q_(z), wherein:

-   -   k is an integer of at least 1;     -   E represents a non-metallic atom selected from oxygen and         selenium;     -   x can be 0 or an integer, and preferably k+x is at least 4, more         preferably in the range of 4 to 10, such as 4 to 7, most         preferably 4 or 7;     -   L represents a ligand that confers oil-solubility or         oil-dispersibility on the molybdenum-sulfur compound, preferably         L is a monoanionic ligand;     -   n is an integer in the range of 1 to 4;     -   A represents an anion other than L, if L is an anionic ligand;     -   p can be 0 or an integer;     -   Q represents a neutral electron-donating compound; and     -   z is in the range of 0 to 5 and includes non-stoichiometric         values.

Those skilled in the art will realise that formation of the trinuclear molybdenum-sulfur compound will require selection of appropriate ligands (L) and other anions (A), depending on, for example, the number of sulfur and E atoms present in the core, i.e. the total anionic charge contributed by sulfur atom(s), E atom(s), if present, L and A, if present, must be −12. The trinuclear molybdenum-sulfur compound may also have a cation other than molybdenum, for example, (alkyl)ammonium, amine or sodium, if the anionic charge exceeds −12.

Examples of Q include water, alcohol, amine, ether and phosphine. It is believed that the electron-donating compound, Q, is merely present to fill any vacant coordination sites on the trinuclear molybdenum-sulfur compound.

Examples of A can be of any valence, for example, monovalent and divalent and include disulfide, hydroxide, alkoxide, amide and thiocyanate or derivative thereof; preferably A represents a disulfide ion.

Preferably, L is monoanionic ligand, such as dithiophosphates, dithiocarbamates, xanthates, carboxylates, thioxanthates, phosphates and hydrocarbyl, preferably alkyl, derivatives thereof. When n is 2 or more, the ligands can be the same or different.

In an embodiment, independently of the other embodiments, k is 4 or 7, n is either 1 or 2, L is a monoanionic ligand, p is an integer to confer electrical neutrality on the compound based on the anionic charge on A and each of x and z is 0.

In a further embodiment, independently of the other embodiments, k is 4 or 7, L is a monoanionic ligand, n is 4 and each of p, x and z is 0.

The molybdenum-sulfur cores, for example, the structures depicted in (I) and (II) above, may be interconnected by means of one or more ligands that are multidentate, i.e. a ligand having more than one functional group capable of binding to a molybdenum atom, to form oligomers. Molybdenum-sulfur additives comprising such oligomers are considered to fall within the scope of this invention.

Other examples of molybdenum containing compounds include molybdenum carboxylates and molybdenum nitrogen complexes, both of which may be sulfurised.

In an embodiment, a molybdenum-containing compound, such as a trinuclear molybdenum dithiocarbamate is preferred.

Boron may also be present in the lubricating oil compositions of the present invention. Boron-containing additives may be prepared by reacting a boron compound with an oil-soluble or oil-dispersible additive or compound. Boron compounds include boron oxide, boron oxide hydrate, boron trioxide, boron trifluoride, boron tribromide, boron trichloride, boron acid such as boronic acid, boric acid, tetraboric acid and metaboric acid, boron hydrides, boron amides and various esters of boron acids. Examples of boron-containing additives include a borated dispersant; a borated dispersant VI improver; an alkali metal or a mixed alkali metal or an alkaline earth metal borate; a borated overbased metal detergent; a borated epoxide; a borate ester; a sulfurised borate ester; and a borate amide. A preferred boron-containing additive is a borated dispersant.

Rust inhibitors selected from the group consisting of nonionic polyoxyalkylene polyols and esters thereof, polyoxyalkylene phenols, and anionic alkyl sulfonic acids may be used.

Copper and lead bearing corrosion inhibitors may be used, but are typically not required with the formulation of the present invention. Typically such compounds are the thiadiazole polysulfides containing from 5 to 50 carbon atoms, their derivatives and polymers thereof. Derivatives of 1,3,4-thiadiazoles such as those described in U.S. Pat. Nos. 2,719,125; 2,719,126; and 3,087,932; are typical. Other similar materials are described in U.S. Pat. Nos. 3,821,236; 3,904,537; 4,097,387; 4,107,059; 4,136,043; 4,188,299; and 4,193,882. Other additives are the thio and polythio sulfenamides of thiadiazoles such as those described in U.K. Patent Specification No. 1,560,830. Benzotriazoles derivatives also fall within this class of additives. When these compounds are included in the lubricating composition, they are preferably present in an amount not exceeding 0.2 wt. % active ingredient.

A small amount of a demulsifying component may be used. A preferred demulsifying component is described in EP 330,522. It is obtained by reacting an alkylene oxide with an adduct obtained by reacting a bis-epoxide with a polyhydric alcohol. The demulsifier should be used at a level not exceeding 0.1 mass % active ingredient. A treat rate of 0.001 to 0.05 mass % active ingredient is convenient.

Pour point depressants, otherwise known as lube oil improvers, lower the minimum temperature at which the fluid will flow or can be poured. Such additives are well known. Typical of those additives which improve the low temperature fluidity of the fluid are C₈ and C₁₈ dialkyl fumarate/vinyl acetate copolymers, polyalkylmethacrylates and the like.

Foam control can be provided by many compounds including an antifoamant of the polysiloxane type, for example, silicone oil or polydimethyl siloxane.

Representative effective amounts of such additives, when used in lubricating oil compositions, are as follows: Mass % a.i.* Mass % a.i.* Additive (Broad) (Preferred) Viscosity Modifier  0.01-6    0.01-4   Corrosion Inhibitor  0.01-5    0.01-1.5  Oxidation Inhibitor  0.01-5    0.01-1.5  Friction Reducer  0.01-5    0.01-1.5  Dispersant  0.1-20   0.1-8   Multifunctional Viscosity Modifier  0.0-5    0.05-5   Detergent  0.01-6    0.01-3   Anti-wear Agent  0.01-6    0.01-4   Pour Point Depressant  0.01-5    0.01-1.5  Rust Inhibitor  0.0-0.5 0.001-0.2  Anti-Foaming Agent 0.001-0.3 0.001-0.15 Demulsifier  0.0-0.5 0.001-0.2  *mass % active ingredient based on the final lubricating oil composition.

The amount of nitrogen derived from the dispersant additive composition typically present in an additive concentrate is in the range of 0.33 to 0.47, such as 0.37 to 0.41, mass %, based on the mass of the additive concentrate.

The amount of soap derived from the detergent additive composition generally present in an additive concentrate is in the range of 103 to 145, such as 116 to 125, millimoles per 1000 g of concentrate.

Thus, a method of preparing a lubricating oil composition according to the present invention can involve admixing an oil of lubricating viscosity and one or more of additives or additive concentrates that comprises two or more of additives and then, optionally, admixing other additive components, such as viscosity modifier, multifunctional viscosity modifier and pour point depressant.

The phosphorus and sulfur content of the lubricating oil composition is advantageously derived from additives in the lubricating oil composition, such as zinc dithiophosphate.

The lubricating oil compositions may be used to lubricate mechanical engine components, particularly an internal combustion, such as a compression-ignited, engine, by adding the lubricating oil thereto. Particular examples of compression-ignited engines are those developed in recent years where the top ring groove temperature may exceed 150, preferably exceed 250, ° C., due to increases in specific power output to around 5 or greater, such as 25 or greater, preferably at least 30, especially 40 or greater, kW/litre. Preferably the maximum specific power output is around 60 kW/litre. These engines are more prone to suffer from ring-sticking problems in their operation.

It should be appreciated that interaction may take place between any two or more of the additives, including any two or more detergents, after they have been incorporated into the oil composition. The interaction may take place in either the process of mixing or any subsequent condition to which the composition is exposed, including the use of the composition in its working environment. Interactions may also take place when further auxiliary additives are added to the compositions of the invention or with components of oil. Such interaction may include interaction which alters the chemical constitution of the additives. Thus, the compositions of the invention include compositions in which interaction, for example, between any of the additives, has occurred, as well as compositions in which no interaction has occurred, for example, between the components mixed in the oil.

In this specification:

The term “hydrocarbyl” as used herein means that the group concerned is primarily composed of hydrogen and carbon atoms and is bonded to the remainder of the molecule via a carbon atom, but does not exclude the presence of other atoms or groups in a proportion insufficient to detract from the substantially hydrocarbon characteristics of the group.

The term “comprising” or “comprises” when used herein is taken to specify the presence of stated features, integers, steps or components, but does not preclude the presence or addition of one or more other features, integers, steps, components or groups thereof. In the instance the term “comprising” or comprises” is used herein, the term “consisting essentially of” and its cognates are a preferred embodiment, while the term “consisting of” and its cognates are a preferred embodiment of the term “consisting essentially of”.

The term “oil-soluble” or “oil-dispersible”, as used herein, does not mean that the additives are soluble, dissolvable, miscible or capable of being suspended in the oil in all proportions. They do mean, however, that the additives are, for instance, soluble or stable dispersible in the oil to an extent sufficient to exert their intended effect in the environment in which the oil composition is employed. Moreover, the additional incorporation of other additives such as those described above may affect the solubility or dispersibility of the additives.

“Major amount” “Major amount” means in excess of 50, such as greater than 70, preferably 75 to 97, especially 80 to 95 or 90, mass %, of the composition.

“Minor amount” means less than 50, such as less than 30, for example, 3 to 25, preferably 5 or 10 to 20, mass %, of the composition mass % of the composition.

The term ‘molybdenum-sulfur compound’ means a compound having at least one molybdenum atom and at least one sulfur atom, preferably the compound has at least one sulfur atom that is bonded to one or more molybdenum atoms and also bonded to one or more non-molybdenum atoms, such as carbon, more preferably the compound has at least one sulfur atom that is bonded to one or more molybdenum atoms only, such as represented by cores [MO₂S₄], [Mo₃S₄] and [Mo₃S₇]. Atoms selected from oxygen and selenium may replace one or more sulfur atoms in such cores. Advantageously, the core consists of molybdenum and sulfur atoms alone. Accordingly, the term ‘molybdenum-sulfur additive’ means an additive comprising one or more molybdenum-sulfur compounds.

All percentages reported are mass % on an active ingredient basis, i.e. without regard to carrier or diluent oil, unless otherwise stated.

The abbreviation SAE stands for Society of Automotive Engineers, who classify lubricants by viscosity grades.

The invention will now be particularly described, by way of example only, as follows:—

EXAMPLES

Lubricating oil compositions meeting the SAE 5W-30 grade were prepared by methods known in art and were subjected to an engine test used to investigate deposit formation, based specifically on the VWTDi CEC-L-78-T-99 test, also known as the PV1452 test. The test is regarded as an industry standard and as a severe assessment of a lubricant's performance capabilities.

With the exception of the dispersant and detergent additive compositions, each lubricating oil composition contained the same additives in the same amount. Oils A, B, C and 1 each contained a dispersant additive composition of borated and non-borated dispersants with Oils C and 1 containing a higher amount of non-borated dispersant. Oils A and C contained a calcium salicylate having a TBN of 168, whilst Oils B and 1 contained a detergent additive composition of a calcium salicylate having a TBN of 168 and a calcium salicylate having a TBN of 64. The basestock used in each lubricating oil composition was a Group III basestock. The properties of the lubricating oil compositions are listed in Table 1.

Tests and Results

The VWTDI test employs a 4-cylinder, 1.9 litre, 81 kW passenger car diesel engine.

It is a direct injection engine, in which a turbocharger system is used to increase the power output of the unit. The industry test procedure consists of a repeating cycle of hot and cold running conditions—the so-called PK cycle. This involves a 30 minute idle period at zero load followed by 180 minutes at full load and 4150 rpm. The entire cycle is then repeated for a total of 54 hours. In this 54 hour period the initial oil fill of 4.5 liters of test lubricant is not topped up.

At the end of the 54 hour test, the engine is drained, the engine disassembled and the pistons rated for piston deposits and piston ring sticking. This affords a result which is assessed relative to an industry reference oil (RL206) to define passing or failing performance.

The pistons are rated against what is known as the DIN rating system. The three piston-ring grooves and the two piston lands that lie between the grooves are rated on a merit scale for deposits and given a score out of 100 by a method known to those skilled in the art. In summary, the higher the number the better the performance: 100 indicates totally clean and 0 indicates totally covered with deposit. The five scores are then averaged to give the overall piston cleanliness merit rating. The scores for each of the four pistons are then averaged to afford the overall piston cleanliness for the test. The results are shown in Table 1 below.

The data in Table 1 show that Oil B and Oil C, each having an increased soap content or dispersant content respectively compared with Oil A, results in an improvement in the cleanliness of the pistons. However, Oil 1 containing an increased soap content and dispersant content provided a significant improvement in the piston cleanliness compared to Oils A, B and C. Thus, use of an increased amount of a dispersant and detergent in lubricating oil composition, which composition gives at most 1.0 mass % of sulfated ash, is useful for achieving a passing performance in the VWTDi engine test. TABLE 1 Oil A Oil B Oil C Oil 1 lowDis/low lowDis/high highDis/low highDis/high Soap Soap Soap Soap sulfated ash, 0.75 0.85 0.75 0.85 mass % TBN, mg 6.67 7.58 7.48 8.30 KOH g⁻¹ Soap derived 15 30 15 30 from detergent, mmol of soap per 1000 g of the oil Nitrogen 0.051 0.051 0.085 0.085 derived from dispersant, mass % Phosphorus, 0.05 0.05 0.05 0.05 mass % Sulfur, mass % 0.16 0.17 0.17 0.18 Boron, ppm 64 64 64 64 Molybdenum, 165 165 165 165 ppm RESULTS VWTDi, merit 57 60 61 66 score (overall piston cleanliness) 

1. A lubricating oil composition comprising, or made by admixing, a major amount of oil of lubricating viscosity, and minor amounts of (A) a dispersant additive composition and (B) a detergent additive composition, wherein the oil composition gives a sulfated ash content of at most 1.0 mass %; has a total base number (TBN) of from about 4 to 9.5; has at least 0.08 mass % of nitrogen derived from the dispersant additive composition, based on the mass of the oil composition; and has at least 25 mmol of soap per 1000 g of the oil composition derived from the detergent additive composition.
 2. An oil composition according to claim 1, wherein the oil of lubricating viscosity comprises a Group III basestock.
 3. An oil composition, according to claim 1, wherein the dispersant additive composition comprises a borated dispersant and non-borated dispersant.
 4. An oil composition, according to claim 1, wherein at most 35% of the mmol of soap in the lubricating oil composition is derived from a detergent having a TBN greater than
 150. 5. An oil composition according to claim 1, wherein the detergent additive composition comprises at least one metal detergent based on one or more organic acid not containing sulfur.
 6. An oil composition according to claim 5, wherein said at least one metal detergent based on one or more organic acid not containing sulphur is a salicylate detergent.
 7. An oil composition according to claim 6, wherein said salicylate detergent is a calcium salicylate detergent having a TBN of, at most,
 150. 8. An oil composition according to claim 5, wherein the detergent additive composition consists of a calcium salicylate having a TBN of 150 to 200 and a calcium salicylate having a TBN of at most
 80. 9. An oil composition according to claim 1, further comprising one or more of an antioxidant composition comprising an aromatic amine, a hindered phenol and, a molybdenum compound.
 10. An oil composition according to claim 1, further comprising a zinc dialkyldithiophosphate containing secondary alkyl groups in a major molar proportion, based on the total amount of alkyl groups.
 11. An oil composition according to claim 1, wherein the oil composition has a sulfur content of at most 0.25 mass %, based on the mass of the oil composition.
 12. An oil composition according to claim 1, wherein the oil composition has a phosphorus content of at most 0.09 mass %, based on the mass of the oil composition.
 13. An oil composition according to claim 9, wherein the oil composition has a molybdenum content of at most 300, based on the mass of the oil composition.
 14. An oil composition according to claim 1, wherein the oil composition has a boron content of at most 150 ppm, based on the mass of the oil composition.
 15. An oil composition, according to claim 1, in which at least one dispersant comprises a polyisobutenyl succinimide prepared using high vinylidene polyisobutene.
 16. A method of lubricating a compression-ignited internal combustion engine comprising operating the engine and lubricating the engine with a lubricating oil composition as claimed in claim
 1. 17. A method of improving piston cleanliness and reducing the ring-sticking tendencies of a compression-ignited internal combustion engine comprising adding to the engine a lubricating oil composition as claimed in claim
 1. 18. A combination comprising the crankcase of a compression-ignited internal combustion engine, and a lubricating oil composition as claimed in claim
 1. 19. A method of improving piston cleanliness in an internal combustion engine comprising lubricating said internal combustion engine with a lubricating oil composition comprising (1) a dispersant additive composition in an amount that provides at least 0.085 mass % of nitrogen and (2) a detergent additive composition in an amount that provides at least 25 mmol of soap per 1000 g of the oil composition, which lubricating oil composition gives a sulfated ash of most 1.0 mass % and has a TBN of 4 to 9.5.
 20. An additive concentrate for preparing a lubricating oil composition comprising an oleaginous carrier fluid, a dispersant additive composition and a detergent additive composition, in such a proportion as to provide a lubricating oil composition as defined in claim 1 when the oil composition contains 10 to 30 mass %, based on the mass of the oil composition, of additives, said additives excluding viscosity modifier and pour point depressant additives. 